1. Field of the Invention
This invention relates generally to a lens configuration, and more specifically, to a lens configuration which is integral with the output window of an enclosure at least partially surrounding a source of a beam.
2. Background Information
The output beam of a laser diode is characterized by the fact that the divergence of the beam in a direction perpendicular to the active layer is typically much greater than that in a direction parallel to the active layer. With reference to FIG. 1, the cross-section 3 of the beam emitted from the active layer 2 of laser diode 1 is illustrated after the beam has travelled a certain distance from the laser diode. As seen, the angle of divergence along an axis 4 which is perpendicular to the active layer, .THETA..sub. , is greater than the angle of divergence along an axis 5 which is parallel to the active layer, .THETA..sub..parallel.. The ratio of the angle of divergence along the perpendicular direction, .THETA..sub. , to that along the parallel direction, .THETA..sub..parallel., is typically between 3:1 and 5:1. In many applications, such as bar code scanning, which utilize laser diodes, it is desirable to at least partially equalize the angles of divergence along the parallel and perpendicular directions, such that the beam, in cross-section, achieves a more circular shape, the advantage being increased flexibility with respect to scanning the beam over a bar code symbol.
Another characteristic of the output beam of a laser diode is astigmatism, according to which the apparent source of the beam, when measured from a direction perpendicular to the active layer, differs from that when measured from a direction parallel to the active layer. With reference to FIG. 2, in which compared with FIG. 1, like elements are referenced with like identifying numerals, the apparent source of the rays along the perpendicular axis 4, identified with reference numeral 6 in the figure, is different from the apparent source of the rays along the parallel axis 5, identified with reference numeral 7. The fact that there is a spacing between these sources, identified with numeral 8 in the figure, is what is known as astigmatism. Again, in the applications such as bar code scanning mentioned above, it is desirable to at least partially eliminate the astigmatism in the beam, the advantage being that the beam waist, when measured along the parallel and perpendicular directions, will occur at about the same location along the optical path of the beam, again providing greater flexibility in scanning the beam over a bar code symbol.
Prior art attempts to at least partially equalize the angles of divergence along the parallel and perpendicular directions have involved the use of aperaturing, the use of an external anomorphic prism pair, or the use of a micro-lens, the latter as described in U.S. Pat. Nos. 5,080,706 and 5,081,639. Each of these approaches, however, is fraught with one or more drawbacks. As to aperaturing, this approach is characterized by low optical efficiency. As to the use of an anomorphic prism pair, this approach is costly and results in a significant increase in the size of a system. As to the use of a micro-lens, this approach requires extremely tight tolerances in the manufacturing process, and is therefore costly.
Accordingly, it is an object of the present invention to provide a lens configuration which achieves greater optical efficiency and reduced cost and size compared to prior art methods. Additional objects and advantages of the invention will be set forth in the description which follows or may be learned by practice of the invention.